Aluminum metal is prepared electrolytically by the reduction of alumina. Conventional alumina reduction cells comprise a vessel having a refractory lining containing, as molten electrolyte, alumina dissolved in fused cryolite. The floor of the cell or vessel typically serves as part of the cathode. At least one anode is disposed within the vessel spaced apart from the cathode. Upon the passage of an electric current between anode and cathode, aluminum is formed by electrolytic reduction of the alumina. The resultant molten aluminum is denser than the cryolite electrolyte and collects as a pool of metal on the floor of the cell.
U.S. Defensive Publication No. T993,002 (Kaplan et al--issued Apr. 1, 1980) discloses the use of a titanium diboride surface to contact molten aluminum at the bottom of an alumina reduction cell. The titanium diboride surface is provided by refractory tiles secured to a carbonaceous substrate. The tiles are stated to be wettable by molten aluminum and to be chemically inert under the conditions of the electrolyte process.
Typically, refractory boride shapes are manufactured by hot pressing in graphite dies at pressures of about 1.5-3.0.times.10.sup.7 Pa and temperatures of 2000.degree.-2050.degree. C. for approximately two hours. On removal from the die and brushing to remove adhering graphite, the hot pressed shape has a grey and rough appearnace.
It has been shown that the surface of the refractory is rich in carbon which is present in all hot pressings made in graphite or carbon dies. When placed in an environment such as that disclosed in the Kaplan et al Defensive Publication, it has become evident that the presence of the carbon-rich surface layer has a deleterious effect on the life of a pressing when brought into contact with molten aluminum. Cracks can start in the surface layer and propagate throughout the hot pressed shape.
Previously it has been proposed that the carbon-rich surface layer should be removed mechanically, such as by diamond grinding. However, this process is very expensive and time consuming since; for example, to remove 1 mm. from all of the surfaces of a titanium diboride plate 250 mm. square by 25 mm. thick, may take about 30 hours.
It has now been found that the surface layer can be removed effectively by alternative means which are both more economical and can be completed in a much shorter time.